Objectives: To identify the conditions necessary for the neoplastic response of mouse salivary gland epithelium to polyoma virus. Emphasis is on epithelio-mesenchymal interaction during tumorigenesis. Major findings: We have obtained extremely interesting results in PV-infected cultures in which epithelium alone was grown on both sides of the millipore filters. In 3 of 19 such cultures, epithelial transformation occurred as judged by morphological and growth-in-vitro criteria. Two of these cultures are being continued after transfer from the filter chambers to Leighton tubes, in attempts to grow them as continuous cell lines. As soon as these transformed lines grow well enough to provide cells for animal inoculation, they will be tested for tumorigenicity in isogeneic mice. Light microscopically, the epithelial cells transformed in the absence of salivary mesenchyme are not distinguishable from those transformed in the presence of salivary mesenchyme. Since last year's report, we have succeeded in obtaining tumors in mice by transplanting epithelial cells transformed in transfilter cultures of epithelium opposite mesenchyme. The morphologic features of these tumors are somewhat variable; of much significance is the fact that the culture-derived tumors are not identical to salivary tumors induced in vivo. They tend to be more highly anaplastic, more invasive, and have less distinct epithelial features than do the tumors induced in vivo. The histological comparisons are not yet complete, however. Proposed course: Present plans are to obtain repeated examples of transformation of salivary epithelium growing on millipore filters in the absence of mesenchyme, and to further characterize the tumors produced by epithelium transformed in this way.